Indefatigable, tireless, weariless, untiring, unwearying, unwearied, unflagging are comparable in their basic meaning of not feeling or manifesting fatigue, but they are closer synonyms in their extended sense of capable of prolonged and arduous effort.
Indefatigable implies being incapable of being fatigued, but in its actual use it usually suggests persistent and unremitting activity or effort.
Tireless and weariless are sometimes employed with little distinction from indefatigable; frequently, however, they connote less busyness and even greater or more remarkable power of continuance.
Untiring and unwearying differ from tireless in carrying a stronger implication of uninterrupted activity; often they specifically suggest an extraordinary ability to go on continuously and without a break while tireless and weariless, by contrast, often imply repeated returns over a very long course of time.
Unwearied differs little from untiring in its meaning, but it is more often applied directly to the person or thing concerned than to the activity engaged in.
Unflagging differs little from tireless, for it too stresses a display of power to continue without signs of weariness; but it also stresses no diminution of activity, and it applies to a person’s powers rather than to the person himself.